I'm a little pedantic about written English.
Spelling, finger trouble and proof reading aside, some very common grammatical mistakes (which I'll try to ignore if spoken) irritate me immensely when written by 'professional' writers.
First of all, when the subject of a sentence is a company (singular), why do so many people refer to it as 'they'?
Even BBC news readers/writers don't seem to know when to use 'fewer' rather than 'less'. Generally, less refers to a single noun and fewer relates to plurals, e.g.
There is less money (is...less);
There are fewer people (are...fewer).
'Compared to' is over-used and wrongly used instead of 'compared with'. The differences are explained well on the Daily Writing Tips website. (If in doubt, use 'compared with'.)
Use 'similar to' and 'different from' (or 'different than' in the USA) but never 'different to'! The word 'alternate' is often wrongly used in place of 'alternative' when describing an option. A full explanation with examples can be read at vocabulary.com.
The use of who/whom doesn't bother me (if in doubt, use who) as
'whom' is generally considered to be one of those words confined to
history, another example being 'unto'. As a guide, however, try associating the word 'whom' with 'him' and 'who' with 'he', either as a substitute in a statement or in the imagined answer to a question.
Some people are confused about when to use 'me' or 'I' in a list but this is a really easy one to work out. Just consider which word you would use if you were alone. It's polite to place yourself at the end of a list but there is no grammatical reason to do that. Here are some examples:
The dog and I walked (you wouldn't say 'me walked');
It was too far for the dog and me to run (you wouldn't say 'it was too far for I to run').
The importance of punctuation was famously illustrated in "Eats Shoots and Leaves" - oops, sorry - "Eats, Shoots and Leaves" by Lynne Truss.
The most common error seems to be the use of an apostrophe in plurals, particularly when using numbers or initials, such as: PCs or 1900s. Notice that I've also dropped the full stops between letters, which has become the norm now, as it makes it much simpler. You can write PCs but if you write P.C.s it becomes very tempting to use an apostrophe to clarify that the 's' isn't part of the mnemonic.
An apostrophe is generally required in place of missing letters (e.g. isn't, don't, can't) or to signify possession. Here are some examples:
Music from the 1960s (no apostrophe; 1960s is plural);
Listen to 1960s' music (plural possessive!)
However, it could be argued that 1960s refers to a decade, singular:
Listen to 1960's music
which can probably be justified (I realise that I've used it this way on several occasions) but it's ambiguous because it could also mean music from the year 1960. I think, in trying to make it clearer, I've just stirred up some muddy waters!
Apostrophe: if in doubt, leave it out! That way, it just seems like you've missed it, which is less embarrassing and less noticeable than erroneous grammar.
The word 'its' is an anomaly because it seems to contradict the general rule. Use it with an apostrophe only to represent 'it is'. Think of 'its' in the same way as 'his' (e.g. his hair is red, its leaves are green).
Of course, there are always exceptions that prove the rules and I make a lot of glaring errors in blogs and on Twitter (often lazily) but I'm willing to improve!